This study will characterize and identify receptors for insulin in arterioles, venules and capillaries of the human and animal retina. Insulin receptors in the retina will be identified, quantitated, and purified. Insulin localization in the retinal capillaries and mural cells, endothelial cells, or basement membrane will be determined by tissue autoradiography. The studies will determine: (a) the nature of the insulin receptors in retinal blood vessels, (b) interactions of insulin with growth hormone, glucagon, epinephrine, at the level of the cyclic nucleotide systems, (c) whether insulin plays a role in glucose and amino acid transport and metabolism through retinal blood vessels, and (d) whether the number of receptors or binding affinity of the insulin receptors is affected in diabetic retinopathy. This information will be useful in the interpretation of the physiology of the retinal vessels and its cells and in the pathogenesis of diabetic retinopathy.